Field of Invention
The present invention relates to fields of technical equipment for oil-gas drilling engineering, mining, geological drilling, construction engineering, tunneling engineering, shield tunneling, trenchless engineering and etc., and more particularly to drill bit with modular cutting-teeth arrangement and having controllable drilling specific pressure.
Description of Related Arts
Bits are tools which directly contact with rocks and break the rocks by effects such as cutting and stamping during drilling process. Polycrystalline Diamond Compact Bits (PDC bits) which break the rocks by cutting and shearing are getting more and more extensive application in drilling engineering, geological engineering and even architectural engineering. The application proportion of the PDC bits in the drilling engineering is getting larger and larger. In a perfect operating condition that a center line of bit coincides with the borehole axis, motion trajectories of all the cutting teeth are relatively fixed concentric-circles during the bit drilling process. Due to rock breaking mechanism and structural differences, the PDC bits are more suitable for soft and medium-hard strata with high ROP (penetration rate). The characteristics of PDC bits include the following.
(1) Manner of Breaking the Rock
The PDC bits cut, shear and break rock depending on the compacts. According to abrasive property of the rocks, continuous or intermittent contacts and frictions are generated between the rocks and the bits during the rock-breaking process.
(2) Relationship Among Cutting-edge Length, Specific Pressure and ROP of the Cutting Element
While the bit is breaking rocks, the bit itself is gradually getting blunt by the abrasion of the rocks simultaneously, i.e., the cutting-edge length is increased. Therefore, the relationship among the cutting-edge length, the specific pressure and the ROP is as following. In the preliminary stage of drilling by the bit, an abrasion line of the cutting element is short, the specific pressure thereof is high, rock-breaking efficiency thereof is high and the ROP thereof is high as well. As the abrasion line thereof is gradually getting longer, the specific pressure thereof becomes lower, relative abrasion of the cutting element to the rocks is increasing, and the ROP thereof is gradually reducing until the bit is failure. For example, if a Φ16 mm circular compact bit serves as the cutting element and is drilling the medium-hard strata, the bit usually becomes failure when the cutting-edge length of the compact is approximately 80% of a diameter thereof. A sharp increase of the cutting-edge length will greatly reduce the breaking efficiency of the bit to the rocks and the drilling efficiency thereof accordingly.
(3) Relationship Among Abrasion Ratio of the Cutting Element, Effective Abrasion Volume and Service Life of the Bits
For a compact having a definite abrasion ratio, the greater the effective abrasion volume, the greater the effective utilization rate of the compact of the bit, and the higher the drilling depth, the longer the service life of the bit. For example, if the Φ16 mm circular compact bit serves as the cutting element and is drilling the medium-hard strata, actual abrasion volume of diamond layer of the compact is less than 20% of the total volume. Thus, without considering the effects of the impact damage, choosing a compact having a high abrasion ratio is conductive to prolonging the service life of the bit.
In order to improve performances of the compact, researcher from China and other countries have studied on several aspects such as cross-sectional shape of the PDC bits, back rake angle, cutting-teeth size and the teeth-arrangement density. However, the problem does not be solved effectively that the actual abrasion volume of the diamond layer is just a very small proportion of the total diamond volume which shorten the bit service life.